Elucidation of the poly-L-proline binding site in Acanthamoeba profilin I by NMR spectroscopy.

The multifunctional protein profilin is one of the most abundant proteins in the cytoplasm and is thought to regulate actin assembly and the phosphoinositide signaling pathway. Profilin binds to several different ligands including actin, poly-L-proline, and the head groups of polyphosphoinositides. Knowledge of profilin/ligand interactions is important for understanding the physiology of profilin in the cell. As a first step in the characterization of profilin/ligand complexes, we have studied a profilin/poly-L-proline complex in solution using high resolution NMR spectroscopy. Analysis of profilin NOE's and chemical shift data indicates that the protein secondary structure is conserved upon binding to poly-L-proline and that the binding site is located between the N- and C-terminal helices in a region rich in highly conserved aromatic sidechains. This site is adjacent to the proposed binding site for actin. In addition, the rate constant for dissociation of the complex is found to be 1.6 +/- 0.2 x 10(4) s-1.